Newspapers are mainly produced in wet offset printing systems. Printing presses, as the present invention preferably pertains to them, typically contain printing mechanisms with rubber blanket cylinders, plate cylinders, inking mechanisms and moistening mechanisms. A printing form tensioned on a printing form cylinder has a surface mostly in the form of a top layer, which has hydrophilic (water-attractive) and hydrophobic (water-repellent) or lipophilic areas in the illustrated state. The printing form is usually formed by a printing plate, which is mounted on a printing form cylinder designed as a plate cylinder. The printing form has applied lipophilic areas for images. The non-image areas are hydrophilic and bind water more intensely than the ink used for printing. The lipophilic areas repel water and therefore have an ink-attractive action. In principle, any surface, which can be divided into hydrophilic and hydrophobic or lipophilic areas, can be used for the offset process.
An illustration is defined below as an operation, in which the printing form is acted on in the areas that form the image dots, so that an original corresponding to the printing image is produced on the printing form by the formation of hydrophilic and lipophilic areas. In terms of the present invention a clearing is defined as an operation, in which the printing form is preferably not treated depending on images, but rather over its entire area, such that the image information applied in the illustration, i.e., the printing image, is removed again.
A plurality of materials and processes exist for producing suitable printing forms or printing plates. It is possible, e.g., to radiate a printing form with a laser for images and then to chemically develop them. Printing forms can also be produced by means of laser ablation. In this case, either lipophilic areas under a hydrophilic layer or hydrophilic areas under a lipophilic layer are uncovered. Also, in terms of an image, material can be applied to a surface, e.g., with an ink jet process or by means of thermodiffusion and an offset printing form can thereby be created. The definitive operation for the illustration of the printing form may be carried out either in a separate facility or within the printing press.
At the moment, printing forms used in mass productions are only used once. However, processes of illustrating printing forms repeatedly and then clearing them again have also become known. In the case of clearing, the surface properties of the printing form are affected, such that the printing form is again made uniformly hydrophilic or hydrophobic over the entire area due to clearing, so that this printing form can be used again for a new illustration operation.
H. Kipphan: Handbuch der Printmedien [Printing Media Manual], Springer Verlag 2000, Chapter 4.4, Computer to Press/Direct Imaging, pages 654–686, gives a review of materials and mechanisms of action for illustrating printing forms by producing different hydrophobic and hydrophilic areas. The materials and processes disclosed there can basically be used in terms of the present invention, so that these materials and processes are expressly involved in this application by way of reference.
It is difficult to achieve high print runs, as they occur, e.g., in the printing of newspapers, with the clearable printing forms made of prior-art materials and with the prior-art illustrating and clearing process because it may occur during the print production that the areas of the printing form made hydrophilic or hydrophobic in an illustration gradually lose these surface properties. As a result, there may be a reduction in the contrast of the printing image. Thus, there may be, e.g., toning, if the hydrophilicity of areas made hydrophilic decreases and these areas take up more and more ink used for the printing.
The aforementioned problems exist not only in wet offset but also in dry offset because in dry offset as well, the printing process is based on the different ink transfer of image-related areas. It is well known that the print quality, especially the print run stability, depends greatly on the maintenance of the production parameters, e.g., the temperature. The ink transfer or repulsion is, indeed, not based on the different hydrophilicity or hydrophobicity of the printing form. Even so, a dry offset printing process is also based on interface processes. Thus, in dry offset as well, the obtaining of the surface properties of a printing form is of decisive importance for achieving high print runs.